1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor and a production method thereof. Specifically, the present invention relates to a photoreceptor in which a photosensitive layer containing an organic material is laminated on a conductive substrate, and a production method thereof.
2. Description of the Related Art
In recent years, a large number of organic electrophotographic photoreceptors made from organic photoconductive materials have been proposed and used in practice as an electrophotographic photoreceptor. This is because the organic electrophotographic photoreceptor is pollution-free and provides cost reduction and flexibility of selection of materials, and therefore various photoreceptor properties can be designed. The photosensitive layer of the organic electrophotographic photoreceptor mainly consists of a layer comprising an organic photoconductive material dispersed in a resin. There have been proposed many photoreceptors having structures such as a lamination structure consisting of a layer in which a charge generation material is dispersed in a resin (a charge generation layer, hereinafter referred to as “CGL”) and a layer in which a charge transporting material is dispersed in a resin (a charge transport layer, hereinafter referred to as “CTL”); a monolayer structure in which a charge generation material (hereinafter referred to as “CGM”) and a charge transporting material (hereinafter referred to as “CTM”) are dispersed in a resin; and others. Of these, a functionally separated photoreceptor comprising a photosensitive layer formed by laminating a charge transport layer on a charge generation layer, is excellent in electrophotographic properties and durability, and is broadly used in practical applications.
In recent years, the miniaturization and speedup of a machine body including both a copying machine and a printer have been required. That is, all the properties of a photoreceptor such as longevity due to improved wear resistance, high sensitivity corresponding to speedup, resistance against hazardous ozone or nitrogen oxides generated by corona discharge and others, are required.
To meet these requirements, an electrophotographic photoreceptor with high sensitivity and excellent resistance to ozone or nitrogen oxides, which is formed of a charge transporting material having great ionization potential, has been studied and practically used.
Therefore, photoreceptor drums for use in both high-speed and low-speed machines are required, and so various types of photoreceptor drums having different properties such as durability or sensitivity need to be produced.
FIG. 1 is a view showing a dip coater used in the production of an electrophotographic photoreceptor. This dip coater is comprised of: a dip coating tank 4 which is filled with a dip coating liquid 5 prepared by dissolving a charge transport substance in a binder resin solution; an auxiliary tank 7 which connects to the dip coating tank 4 via a pump 6; an elevating machine 2 which moves a cylindrical conductive substrate 1 up and down; and a motor 3. As the dip coating liquid 5 is consumed in the dip coating tank 4, the dip coating liquid 5 pooled in the auxiliary tank 7 is supplied from a dip coating liquid supply port 14 to the dip coating tank 4 via the pump 6. When the dip coating liquid overflows the dip coating tank 4, it is received in an overflow tank 13 and is then transported to the auxiliary tank 7. The dip coating liquid 5 pooled in the auxiliary tank 7 is monitored for viscosity by a viscosity measuring device 10. Then, to maintain uniform viscosity, a dilution pooled in an addition solvent tank 9 is added to the dip coating liquid 5, and the mixture is stirred with an agitator 8. The cylindrical conductive substrate 1 is chucked by a cylindrical conductive substrate grasping part 11 and is moved in a vertical direction at a predetermined speed by the elevating machine 2 which comprises the motor 3. To form a photosensitive layer, the conductive substrate 1 is taken down and is immersed in the dip coating liquid 5 pooled in the dip coating tank 4 through the dip coating tank opening port 12. The well-dipped conductive substrate 1 is pulled out of the dip coating tank 4 by the elevating machine 2, so that a photosensitive layer is formed.
Where two or more types of electrophotographic receptors are produced by this dip coater using different charge transporting materials, it would be better if the dip coating tank 4, the auxiliary tank 7 and a circulating device such as a piping or pump could be prepared specifically for each of different charge transporting materials. But the fact is, considering cost reduction, various types of photoreceptor drums with different properties are produced in a single device. Accordingly, when a dip coating liquid is exchanged, a washing operation is required, in which the dip coating liquid used in the previous production is discharged, a washing solvent is poured and circulated in the dip coater, and the washing liquid is then discharged.
At this time, if the apparatus is completely disassembled, and hand wiping is then carried out using a cloth dampened with a washing solvent, the washing level can be raised. However, this washing operation requires considerable time and labor costs, and in fact some portions such as a pump or motor are incapable of being disassembled. Thus, the dip coating liquid used in the previous production inevitably remains. Moreover, when the circulation and discharge of a washing solvent is repeated, the washing level is raised on one hand, but a large amount of washing solvent and time are required on the other.
Japanese Patent Laid-Open No. 9-230614 proposes that, in an electrophotographic photoreceptor, the content of aromatic primary amine in a photosensitive layer thereof is set at 30 ppm or lower with respect to a charge transporting material having a group represented by the following general formula in a molecule thereof: however, the allowance of impurities as a whole is not described in this publication.
The use of various types of photosensitive layers corresponding to different models in the production of an electrophotographic photoreceptor leads to the performance of dip coating using various types of dip coating liquid. Consequently, the circulating system of a dip coating liquid such as a dip coating tank or dip coating liquid agitating wagon, which is used in a production line, is cleaned and maintained, and by exchanging the dip coating liquids, dip coating is carried out using each dip coating liquid. However, due to the exchange of dip coating liquid, in some cases, the thus produced electrophotographic photoreceptor cannot satisfy the required electric property. Even though electrophotographic receptors are produced under the same conditions, there is variation of the electric property between lots. In the case where such a photoreceptor is mounted, problems occur such that the surface potential VL of the photoreceptor increases after laser exposure and image concentration decreases.
For example, in the production of a photoreceptor (1) there was a great difference regarding electric property between a case where the photoreceptor (1) was produced after a photoreceptor (2) was produced, and a case where the photoreceptor (1) was produced after a photoreceptor (3) was produced. The results are shown in FIG. 2. The original electric property of the photoreceptor (1) was identical to that of the receptor (1) which was produced after the receptor (2), but where the receptor (1) was produced after the receptor (3), the surface potential was significantly deteriorated.
It was considered that some dip coating liquid remains in some portions incapable of being disassembled such as a filter or piping portion when the dip coating liquid for a charge transport layer is exchanged and maintained, and that the deterioration (increase) of the surface potential VL of the photoreceptor (1) produced after the photoreceptor (3) results from the mixing of such a residual dip coating liquid into the photoreceptor (3). As a result of studies, even where a small amount of CTM of the photoreceptor (3) was added in the charge transport layer of the photoreceptor (1), the deterioration of the surface potential appeared. However, when the CTM of the photoreceptor (2) was added therein at the same ratio, almost no deterioration appeared. As a result of further studies, it was found that the significant deterioration of surface potential occurs when CTM having a smaller ionization potential is added.
A charge transporting material, which is excellent in resistance to ozone or nitrogen oxides, is highly sensitive and has high ionization potential, has come to be used. Because of this, a charge transporting material with conventional low ionization potential used in dip coating in the previous production is likely to be mixed into a dip coating liquid for a charge transport layer, which comprises, as a constitutive substance, the above described material with high ionization potential, and then the material with low ionization potential is likely to act as charge traps. By this phenomenon, it is considered that the sensitivity of a photoreceptor drops and the deterioration of image concentration occurs.
Therefore, when a dip coating liquid is exchanged, a production apparatus needs to be fully washed so that the charge transporting material used in the previous production does not remain. However, a large amount of washing solvent is needed to raise the washing level and this leads to high cost, and further, as described above, some portions cannot be disassembled. Thus, the full washing of a production apparatus is extremely difficult.